The present invention relates to telephone subscriber instruments; and more particularly, it relates to speakerphone instruments.
Speakerphone instruments are well known, e.g., the Bell 4A unit. This type of communication instrument, while providing hands-free telephonic communication capability to a user, suffers from poor quality in the voice transmission. Specifically, the voice transmission from such units carries an echo which is particularly discernible to the listener. The echo results because of the many paths over which the acoustic energy of the user's speech travels in reaching the microphone.
In an effort to improve the quality of voice transmission in speakerphone instruments, several approaches to solving the echo problem have been tried. These approaches include the use of elaborate acoustic and electronic echo-cancelling techniques, such as those developed for satellite communications. The cost of implementing such techniques in a telephone subscriber instrument are prohibitively expensive.
The most effective solution to the echo problem has been to merely place the microphone in close proximity to the user during talking. In order to place the microphone in proximity to the user, it has been suggested to use a boom-mounted microphone or to use a long microphone cord. Both approaches suffer apparent disadvantages. Another suggestion has been to use a wireless microphone.
Wireless microphones can be implemented using either a radio frequency (RF) transmission link or a diffuse optical radiation transmission link, i.e., an infrared (IR) link. An RF link is unsuitable because of the inability to maintain privacy in communications due to the difficulty to restrict the spread of an RF transmission. Additionally, an RF link is susceptible to interference. An IR link is preferable to an RF link in the aspects of privacy and interference problems, but the power consumption is substantially greater.